A method for excavating a railway low-clearance open-trench tunnel by underpassing a steel box girder

Abstract

The application discloses a kind of underpass steel box girder underpinning railway low clearance open cut tunnel excavation method, belong to existing line railway construction technical field, in the preset reinforcement range of existing railway, the construction of existing railway support structure is carried out;Adjust the sleeper spacing in the preset reinforcement range of existing railway, and make steel box girder system;Carry out stratified excavation in design area, after first layer earthwork excavation is finished, construction concrete support structure in the range of first layer earthwork excavation;During lower layer earthwork excavation, support is carried out using steel structure support system;Steel structure support system is equipped with axial force servo system, and the deformation data of enclosure structure is monitored in real time;Make tunnel main structure;The application applies steel structure support system and servo system to underpass railway low clearance foundation pit, and realizes the dynamic adjustment of supporting axial force by data driving, is an active deformation control means, compared with traditional passive support, can more effectively control the deformation of foundation pit and upper railway structure.

Description

Technical Field

[0001] This invention relates to the field of existing railway construction technology, specifically to a method for excavating a low-clearance open-cut tunnel for railway underpasses with steel box girders. Background Technology

[0002] With the rapid development of urban transportation networks, the number of projects involving the construction of new underground tunnels or passageways under existing operational railway lines is increasing. Among the various construction methods, the cut-and-cover method is the preferred option when conditions permit due to its relatively simple process, high structural reliability, and large construction space. However, when carrying out underpass construction in bustling urban areas or areas with dense existing facilities, severe space constraints and deformation control challenges are often encountered.

[0003] When a new tunnel needs to pass under an existing railway line, and the clearance under the railway bridge is low (i.e., low clearance conditions), traditional cut-and-cover tunnel construction methods reveal many limitations. Specifically: Existing railways face extremely high requirements for deformation control: they are highly sensitive to uneven settlement and horizontal displacement of the track. During the excavation of the foundation pit, the release of soil stress inevitably leads to deformation of the retaining structure and surrounding soil, which in turn causes displacement of the overlying railway track. Once the deformation exceeds the limit, it will directly threaten the operational safety of trains. Traditional methods often employ passive support systems (such as steel or concrete supports with fixed axial force). However, the support force cannot be adjusted according to the real-time deformation state of the retaining structure. Often, insufficient initial pre-applied axial force or increased soil pressure leads to "inadequate support," or excessive axial force has an adverse effect on the structure. Essentially, this is a "lagging" passive control strategy.

[0004] Low clearance conditions limit conventional reinforcement and support operations: Under conditions of limited clearance beneath railway bridges (e.g., only a few meters in height), large machinery cannot easily access the site, and conventional retaining structure construction, support installation and dismantling, and earthwork excavation all face the challenge of limited space. For example, traditional reinforcement equipment such as tall bored piles and high-pressure jet grouting piles may not be able to enter the site, or may not be able to be erected vertically. Traditional steel supports require considerable space for installation and dismantling, and the number of vertical layers of supports is strictly limited by clearance, which may not provide sufficient support rigidity.

[0005] Coordinated Deformation Control of Crossbeam and Longitudinal Beam Systems under Low Clearance Conditions: To address railway line reinforcement issues, existing technologies employ a "crossbeam + longitudinal beam" composite structure to suspend the existing railway, transferring train loads to the supporting structures on both sides of the excavation pit. However, under low clearance conditions, the spatial relationships between the longitudinal and crossbeams, the pit retaining structure, and the internal supports are extremely complex. When the excavation of the pit causes deformation of the retaining structure, it directly affects the smoothness of the overhead railway track through the longitudinal and crossbeams. If the lateral deformation of the pit retaining structure cannot be effectively controlled, even with an elevated system of longitudinal and crossbeams, the deformation of the railway structure will still be insufficient to meet operational safety requirements.

[0006] Currently, for foundation pits with high deformation control requirements, a technology using "steel support + axial force servo system" has been adopted. This involves applying or unloading axial force to the steel support in real time through a hydraulic system to actively control the deformation of the retaining wall. However, this technology is mostly applied to subway stations with conventional clearance or deep foundation pits, and its control targets are mainly the foundation pit itself and surrounding buildings. There are no publicly available engineering practices or technical solutions that systematically apply this technology to open-cut tunnel foundation pits with low clearance under railways and coordinate deformation control with the "crossbeam + longitudinal beam" overhead system of the overlying railway.

[0007] Therefore, how to safely and efficiently excavate open-cut tunnels under the harsh spatial conditions of low clearance in railways, and how to achieve active and precise control over the deformation of the foundation pit retaining structure and the railway structure above it, is a technical problem that urgently needs to be solved by those skilled in the art. Summary of the Invention

[0008] To address the aforementioned issues, this invention provides a method for excavating a low-clearance open-cut tunnel under a steel box girder supporting a railway. This method applies a steel structure support system and a servo system to the low-clearance foundation pit under the railway, and achieves dynamic adjustment of the support axial force through data-driven methods. This is an active deformation control method that can more effectively control the deformation of the foundation pit and the railway structure above compared to traditional passive support.

[0009] To achieve the above objectives, the present invention provides the following solution: A method for excavating a low-clearance open-cut tunnel under a steel box girder railway includes the following steps: Step 1: Within the pre-designated reinforcement area of ​​the existing railway, construct the supporting structure for the existing railway. Step 2: Adjust the sleeper spacing within the pre-set reinforcement range of the existing railway, insert crossbeams between the sleepers, install longitudinal beams on the support structure, and connect the crossbeams and the longitudinal beams; Step 3: Before excavating the foundation pit, construct the foundation pit retaining structure; Step 4: Excavate the foundation pit in layers within the design area. After the first layer of earthwork is excavated, construct a concrete support structure within the excavation area of ​​the first layer. During the excavation of the next layer of earthwork, excavate and support simultaneously, using a steel structure support system. The steel structure support system is equipped with an axial force servo system to monitor the deformation data of the retaining structure in real time and automatically adjust the axial force of the steel structure support system. Step 5: Construct the main tunnel structure; Step Six: Remove the crossbeams and longitudinal beams, and restore the existing railway.

[0010] Preferably, the construction of the support structure includes the following steps: constructing support units on both sides of the existing railway, with the support units evenly spaced, and the construction of the support units includes pile foundation construction and the construction of pile caps on the pile foundations, with longitudinal beams placed on the pile caps.

[0011] Preferably, the construction of the foundation pit retaining structure includes the following steps: constructing a water-stop curtain at the edge of the foundation pit excavation area, and then constructing retaining piles.

[0012] Preferably, the concrete support structure includes a transversely arranged concrete support beam and a trestle plate disposed on the upper part of the concrete support beam, wherein both ends of the concrete support beam and both ends of the trestle plate are connected to the retaining piles.

[0013] Preferably, the steel structure support system includes a steel waler arranged along the circumference of the retaining piles and a steel pipe arranged laterally, with both ends of the steel pipe disposed on the steel waler.

[0014] Preferably, the construction of the main tunnel structure includes the following steps: after excavation to the design elevation of the bottom of the foundation pit, the tunnel bottom slab, side walls and top slab structure are poured from bottom to top. During the construction of the side walls, the corresponding steel structure support system is removed according to the design requirements.

[0015] Preferably, both the crossbeam and the longitudinal beam are D-type temporary beams.

[0016] Preferably, the construction of the water-stop curtain adopts the ultra-high pressure jet grouting method. The water-stop curtain is constructed obliquely or vertically between piles around the foundation pit before excavation. Before the construction of the water-stop curtain, test piles are carried out to ensure that the pile diameter, strength and verticality meet the design requirements under low clearance and limited angle.

[0017] Preferably, the entire process is automated, and the deformation of existing railway tracks, settlement of piers, displacement of retaining structures and axial force of supports are monitored in real time. The monitoring data is directly linked to the steel support servo system.

[0018] Preferably, during the construction of the foundation pit retaining structure, a modified low-headroom drilling rig is used for the construction of retaining piles.

[0019] The present invention achieves the following technical effects compared to the prior art: 1. This invention applies a steel structure support system and a servo system to a low-clearance foundation pit under a railway. By using data-driven dynamic adjustment of the support axial force, it is an active deformation control method that can more effectively control the deformation of the foundation pit and the railway structure above compared to traditional passive support.

[0020] Other technical solutions of the present invention also achieve the following effects: 2. By constructing a high-strength steel box girder support system, open-cut construction below was achieved under conditions of uninterrupted railway operation and no speed limit (or speed restriction). The impact of the foundation pit excavation on the railway above was minimized, and safety risks were controllable. Attached Figure Description

[0021] To more clearly illustrate the technical solutions in this invention or the prior art, the drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0022] Figure 1 This is a schematic diagram of the support structure in the method of the present invention; Figure 2 This is a schematic diagram showing the relative positional relationship between the supporting structure and the foundation pit in the method of this invention; The components include: 1. Existing railway; 2. Longitudinal beams; 3. Pile foundations; 4. Piles; 5. Trellise decks; 6. Crown beams; 7. Water-stop curtains; 8. Foundation pits; 9. Concrete support structures; and 10. Steel structure support systems. Detailed Implementation

[0023] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0024] This invention provides a method for excavating a low-clearance open-cut tunnel under a steel box girder railway. It applies a steel structure support system and a servo system to the low-clearance foundation pit under the railway, and realizes dynamic adjustment of the support axial force through data drive. This is an active deformation control method, which can more effectively control the deformation of the foundation pit and the railway structure above compared with traditional passive support.

[0025] To make the above-mentioned objects, features and advantages of the present invention more apparent and understandable, the present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0026] refer to Figures 1 to 2A method for excavating a low-clearance open-cut tunnel under a steel box girder railway includes the following steps: Step 1: Constructing the support structure of the existing railway 1 within the pre-set reinforcement range of the existing railway 1; Step 2: Adjusting the sleeper spacing within the pre-set reinforcement range of the existing railway 1, inserting crossbeams between the sleepers, setting longitudinal beams 2 on the support structure, and connecting the crossbeams and longitudinal beams 2; Step 3: Constructing the retaining structure of the foundation pit 8 before excavation; Step 4: Excavating the foundation pit 8 in layers within the design area. After the first layer of earthwork is excavated, constructing a concrete support structure 9 within the excavation range of the first layer of earthwork; during the excavation of the next layer of earthwork... The excavation and support were carried out simultaneously, using a steel structure support system 10. The steel structure support system 10 is equipped with an axial force servo system to monitor the deformation data of the retaining structure in real time and automatically adjust the axial force of the steel structure support system 10. Step 5: Construct the main tunnel structure. Step 6: Remove the crossbeams and longitudinal beams 2 and restore the existing railway 1. Specifically, the pile foundation 3 is constructed using a full casing construction method. Within the predetermined "maintenance window" time, the rail fasteners are removed and the sleeper spacing is adjusted. First, the crossbeams are inserted in sequence, then the longitudinal beams 2 are hoisted, and the longitudinal and crossbeams are connected into a whole to form a "steel box girder + rail fastening" joint load-bearing support system. This system spans across the excavated pit 8, transferring the railway load to the pile caps 4 and support piles on both sides, providing a safe low-clearance working space for the excavation of the pit 8 below. This invention applies a steel structure support system 10 and a servo system to a low-clearance foundation pit 8 under a railway. By using data-driven dynamic adjustment of the support axial force, it is an active deformation control method that can more effectively control the deformation of the foundation pit 8 and the railway structure above it compared to traditional passive support.

[0027] Furthermore, when the foundation pit is deep, a concrete support structure 9 can be added to the steel structure support system 10.

[0028] Furthermore, the construction of the supporting structure includes the following steps: supporting units are constructed on both sides of the existing railway 1, with the supporting units evenly spaced. The construction of the supporting units includes the construction of pile foundations 3 and the construction of pile caps 4 on the pile foundations 3. The longitudinal beams 2 are set on the pile caps 4. Specifically, the positions of the pile foundations 3 on both sides of the existing railway 1 correspond one-to-one, and the central axes of the pile caps 4 on the same side are on the same straight line to ensure the stability of the longitudinal beams 2 under stress. In addition, the longitudinal beams 2 erected on the pile caps 4 are detachably connected to the pile caps 4. Rigid plate structures can be pre-embedded on the pile caps 4 and exposed on the surface of the pile caps 4. The longitudinal beams 2 can be directly welded to the rigid plate structures, or locked by a semi-circular buckle set on the rigid plate structures, thereby achieving the purpose of locking the longitudinal beams 2. Furthermore, if the longitudinal beams 2 are locked by the semi-circular buckles, the gap between the inner wall of the semi-circular buckle and the outer wall of the longitudinal beams 2 is filled with square timber to further fix the longitudinal beams 2 and prevent the longitudinal beams 2 from slipping.

[0029] Furthermore, the construction of the retaining structure for the foundation pit 8 includes the following steps: constructing a water-stop curtain 7 at the edge of the excavation area of ​​the foundation pit 8, and then constructing retaining piles; specifically, the excavation edge of the foundation pit 8 mentioned here refers to the edge area of ​​the designed foundation pit 8. After the retaining piles are constructed, the vertical line of the side of the retaining pile closest to the foundation pit 8 should coincide with the edge line of the foundation pit 8; a capping beam 6 is constructed on the top of the retaining piles to form a stable overall structure, thereby further enhancing the stability of the foundation pit 8 support.

[0030] Furthermore, the concrete support structure 9 includes a transversely arranged concrete support beam and a trestle plate 5 set on the upper part of the concrete support beam. Both ends of the concrete support beam and both ends of the trestle plate are connected to the retaining piles. The trestle plate 5 forms a rigid isolation with the support structure above, which can both distribute the upper load and serve as a working platform for subsequent construction. In addition, a stable and reliable initial concrete support structure 9 support system is established in the upper part of the relatively shallow and spacious foundation pit 8 to ensure the stability of the upper part of the foundation pit 8 and the initial safety of the overhead railway system above.

[0031] Furthermore, the steel structure support system 10 includes steel walers arranged along the circumference of the retaining piles and transversely arranged steel pipes, with both ends of the steel pipes mounted on the steel walers. The purpose of placing the steel structure support system 10 below the concrete support system is to address the challenges of large concrete supports that cannot be installed or dismantled and the limited construction space under low clearance conditions. At the same time, through the real-time monitoring and automatic axial force adjustment functions of the system, active and dynamic control of the deformation of the retaining structure can be achieved, thereby accurately constraining the lateral displacement caused by deep soil excavation, effectively protecting the existing railway structure above from excessive settlement or deformation, and ensuring the safety of train operation.

[0032] Furthermore, the installation of the steel structure support system 10 and the earthwork excavation adopt the "unit advancement method", with 2-3 support spacing as a unit, and complete all the procedures of "excavation-support-application of prestress" of a unit within 12-16 hours, making full use of the time and space effect.

[0033] Furthermore, the construction of the main tunnel structure includes the following steps: After excavation to the design elevation of the bottom of pit 8, the tunnel floor slab, side walls, and roof slab are poured sequentially from bottom to top. During the construction of the side walls, the corresponding steel structure support system 10 is removed according to the design requirements. Specifically, after pit 8 is excavated to the design elevation, the foundation is inspected to confirm that the bearing capacity and geological conditions meet the design requirements. Subsequently, the tunnel floor slab cushion concrete (usually plain concrete or low-grade concrete) is promptly poured to provide a flat and dry working surface for subsequent bottom slab reinforcement binding and concrete pouring, and to protect the foundation and prevent disturbance. After the cushion layer reaches a certain strength, the tunnel floor slab reinforcement is bound, formwork is erected, and waterproofing layer construction (such as applying waterproof coating or laying waterproof membrane) is carried out. After acceptance, the tunnel floor slab concrete is poured in one go or in sections continuously, and appropriate curing measures are adopted to control temperature cracks in the large volume concrete. The bottom slab structure is the load-bearing component at the bottom of the tunnel body. Its completion provides a stable foundation for the sidewall construction and begins to bear part of the lateral earth pressure from the retaining structure. After the bottom slab concrete reaches its design strength, the tunnel sidewall construction begins. The sidewall construction and the dismantling of the steel structure support system 10 follow the principle of "from bottom to top, dismantling before pouring, layered and step-by-step, and smooth transition of stress": First, the first (lowest) steel structure support (such as steel bracing) closest to the bottom slab is dismantled. Before dismantling, the axial force is actively unloaded to a safe threshold using an axial force servo system, followed by mechanical dismantling. After dismantling, the reinforcement binding, formwork erection, and concrete pouring of this section of the sidewall are immediately carried out, so that the newly poured sidewall forms an integral whole with the bottom slab, replacing the support function of the dismantled steel bracing and bearing the earth and water pressure transmitted by the retaining structure; after the lower sidewall concrete reaches its design strength, the previous steel bracing is dismantled in the same way, and then the next section of the sidewall is constructed. In this way, "dismantling one layer, pouring one layer, strengthening one layer", the construction proceeds layer by layer from bottom to top until all sidewall construction is completed. During this process, ensure that the deformation of the retaining structure is always under control to avoid instability of the foundation pit 8 or excessive deformation of the railway structure above due to excessively rapid or uneven removal of supports.

[0034] Furthermore, both the crossbeams and longitudinal beams 2 are D-type temporary beams. The D-type temporary beam is a standardized and modular railway overhead equipment specifically designed for the reinforcement of existing railway lines. It has high structural strength and rigidity, and can effectively bear and transmit the operating load of trains. At the same time, its crossbeams and longitudinal beams 2 are reliably bolted together to form a stable spatial frame system, which can adapt to the needs of rapid installation and dismantling under low clearance conditions and can be erected without large lifting equipment. In addition, the D-type temporary beam system has excellent deformation control capability of the track when the railway is overhead. It can cooperate with the active deformation control strategy of "steel support + servo system" for the lowered foundation pit 8 to ensure that the geometric smoothness and operational safety of the existing railway line 1 above are not affected during the entire process of excavation of foundation pit 8 and tunnel construction.

[0035] Furthermore, the construction of the water-stop curtain 7 adopts the ultra-high pressure jet grouting method. Before the excavation of the foundation pit 8, the water-stop curtain 7 is constructed in an inclined or vertical manner between piles. Before the construction of the water-stop curtain 7, test piles are carried out to ensure that the pile diameter, strength and verticality meet the design requirements under low clearance and limited angle.

[0036] Furthermore, the entire process is automated, and the deformation of the existing railway track, the settlement of the pier 4, the displacement of the retaining structure and the axial force of the support are monitored in real time. The monitoring data is directly linked to the steel support servo system.

[0037] Furthermore, during the construction of the retaining structure of foundation pit 8, a modified low-headroom drilling rig was used for the construction of retaining piles, and a segmented hole-forming process of "lifting the drill - connecting the rod - re-drilling" was adopted; the steel cage was made in sections (such as 3 meters per section), and a self-locking sleeve was used for quick connection at the hole opening.

[0038] Taking the Baoshan Station East Throat Area Underpass Tunnel Project of Baosteel Special Line in a certain city as an example, this invention provides a detailed description of the low-clearance open-cut tunnel excavation method for underpass steel box girder railway replacement provided by the present invention.

[0039] Project Background: An existing Class II railway, designated 1, requires the construction of a new open-cut tunnel beneath it, measuring 56.9m long, 17.9m wide, and 16m deep. The clearance for the dedicated railway line above the tunnel is only 5.5m, and railway operations must not be interrupted during construction. The foundation pit (8) is classified as Class I in both safety and environmental protection, with geology primarily consisting of silty clay and muddy clay, and a high groundwater level.

[0040] Specific implementation steps: Constructing a steel box girder support system Support pile construction: At the designed locations on both sides and between lines of the dedicated line, rotary drilling rigs, in conjunction with a full casing follow-up process, are used to construct D-type temporary beam support piles. The full casing provides continuous wall protection to prevent railway subgrade settlement caused by drilling.

[0041] Foundation 4 casting: Eight D-shaped temporary beam foundations 4 with dimensions of 3.5m×0.95m×1.2m are cast on top of the supporting piles to serve as support points for the D-shaped temporary beams.

[0042] Erection of Type D Temporary Beams: Apply for railway construction "maintenance windows" and organize the use of truck cranes to hoist the D16+D24+D16 type temporary beams in sections. First, manual labor combined with small machinery is used to remove ballast and adjust sleeper spacing. Each sleeper is then inserted into the crossbeam, followed by the symmetrical hoisting of the longitudinal beams 2. Finally, high-strength bolts are used to connect the longitudinal and transverse beams. 25m sections of 3-5-3 rails are attached to each side of the temporary beam, forming a combined support system with a total length of 106m. The clearance of this system is 5.5m.

[0043] Construction of low headroom enclosure structure Low-headroom retaining piles: Within a 5.5m clearance below the D-type temporary beam, a modified positive circulation drilling rig with a height of 5.3m is used. After the drilling rig is in place, a hole diameter of 1.2m and a depth of 31.7m are formed using the "lifting drill-connecting rod-re-drilling" method. The drilled pile reinforcement cage is segmented (3m sections) and connected at the wellhead using self-locking sleeves. Concrete is poured using the tremie method, with a low-headroom hopper.

[0044] MJS Water-Cutting Curtain 7: On the outside of the retaining piles, the MJS method equipment is used to construct the water-cutting curtain 7 between the piles, with a pile diameter of 1.8m. By adjusting the drilling rig inclination angle, inclined or vertical drilling is completed in the limited space below the D-shaped temporary beam, and high-pressure jet grouting is performed to effectively seal the gaps between the retaining piles and control the groundwater level outside the foundation pit 8.

[0045] Low-headroom foundation pit 8 Excavation and support First-level excavation and trestle slab 5: Excavate the first layer of soil to the bottom elevation of the first concrete support. Pour the cap beam 6, concrete supports, and 20cm thick trestle slab 5. Trestle slab 5 and the upper D-shaped temporary beam form a composite load-bearing system, enhancing overall rigidity and providing a working platform for machinery.

[0046] Lower-level excavation and steel support servo system: After the concrete supports reach their strength, excavation will proceed in four layers (3-3.5m per layer). The principle of "from the middle to both ends, layer by layer and segment by segment, support first and then excavate" will be adopted.

[0047] The second and third steel supports use ∅609mm (t=16mm) steel pipes, and the fourth support uses ∅800mm (t=20mm) steel pipes.

[0048] When the excavation reaches 0.5m below each support, immediately install the steel waler and steel pipe support.

[0049] Each steel support is equipped with an axial force servo system. During excavation, inclination data of the retaining structure is collected in real time. When the deformation rate or cumulative value triggers an early warning, the system automatically or manually instructs hydraulic jacks to apply loads in stages to compensate for the axial force. For example, when the daily deformation of the retaining wall exceeds 2mm, the system will apply additional pressure to the support according to a preset program (such as increasing by 200kN each time) until the deformation converges.

[0050] Main structure construction and system conversion Structural construction sequence: After excavation to the foundation (16m deep), the foundation layer and waterproof layer are poured, and then the tunnel floor slab, sidewalls, and roof slab are constructed sequentially from bottom to top. A tripod large formwork system is used for the sidewalls. The steel support structure is removed simultaneously during sidewall construction.

[0051] Railway restoration: After the main structure concrete reaches its design strength, another railway "maintenance window" will be applied for. The D-type temporary beams and rail fasteners will be removed in reverse order, the ballast will be backfilled in layers and compacted, and the dedicated line will be restored. After acceptance by Baosteel's operations department, normal train operation will resume.

[0052] Through the above implementation methods, this approach successfully completed the construction of a 16-meter-deep, large-section open-cut tunnel without interrupting railway operations and under conditions of only 5.5 meters of clearance. Practice has proven that this method is safe, reliable, and technologically advanced, possessing extremely high reference value and promotional significance for similar projects.

[0053] It should be noted that, for those skilled in the art, it is obvious that the present invention is not limited to the details of the exemplary embodiments described above, and that the invention can be implemented in other specific forms without departing from the spirit or essential characteristics of the invention. Therefore, the embodiments should be considered exemplary and non-limiting in all respects, and the scope of the invention is defined by the appended claims rather than the foregoing description. Thus, all variations falling within the meaning and scope of equivalents of the claims are intended to be included within the invention. No reference numerals in the claims should be construed as limiting the scope of the claims.

Claims

1. A method for excavating a low-clearance open-cut tunnel under a steel box girder railway, characterized in that, Includes the following steps: Step 1: Within the pre-designated reinforcement area of ​​the existing railway, construct the supporting structure for the existing railway. Step 2: Adjust the sleeper spacing within the pre-set reinforcement range of the existing railway, insert crossbeams between the sleepers, install longitudinal beams on the support structure, and connect the crossbeams and the longitudinal beams; Step 3: Before excavating the foundation pit, construct the foundation pit retaining structure; Step 4: Excavate the foundation pit in layers within the design area. After the first layer of earthwork is excavated, construct the concrete support structure within the area of ​​the first layer of earthwork excavation. During the excavation of the lower layer of earthwork, excavation and support are carried out simultaneously, and a steel structure support system is used for support; the steel structure support system is equipped with an axial force servo system to monitor the deformation data of the retaining structure in real time and automatically adjust the axial force of the steel structure support system; Step 5: Construct the main tunnel structure; Step Six: Remove the crossbeams and longitudinal beams, and restore the existing railway.

2. The method for excavating a low-clearance open-cut tunnel under a steel box girder railway as described in claim 1, characterized in that, The construction of the support structure includes the following steps: support units are constructed on both sides of the existing railway, with the support units evenly spaced. The construction of the support units includes pile foundation construction and the construction of pile caps on the pile foundations, with longitudinal beams placed on the pile caps.

3. The method for excavating a low-clearance open-cut tunnel under a steel box girder railway as described in claim 1, characterized in that, The construction of the foundation pit retaining structure includes the following steps: constructing a water-stop curtain at the edge of the foundation pit excavation area, and then constructing retaining piles.

4. The method for excavating a low-clearance open-cut tunnel under a steel box girder railway as described in claim 3, characterized in that, The concrete support structure includes a transversely arranged concrete support beam and a trestle plate set on the upper part of the concrete support beam. Both ends of the concrete support beam and both ends of the trestle plate are connected to the retaining piles.

5. The method for excavating a low-clearance open-cut tunnel under a steel box girder railway as described in claim 3, characterized in that, The steel structure support system includes steel walers arranged along the circumference of the retaining piles and transversely arranged steel pipes, with both ends of the steel pipes mounted on the steel walers.

6. The method for excavating a low-clearance open-cut tunnel under a steel box girder railway as described in claim 1, characterized in that, The construction of the main tunnel structure includes the following steps: After excavating to the design elevation at the bottom of the foundation pit, the tunnel bottom slab, side walls and top slab structure are poured from bottom to top. During the construction of the side walls, the corresponding steel structure support system is removed according to the design requirements.

7. The method for excavating a low-clearance open-cut tunnel under a steel box girder railway as described in claim 1, characterized in that, Both the crossbeams and the longitudinal beams are D-type temporary beams.

8. The method for excavating a low-clearance open-cut tunnel under a steel box girder railway as described in claim 1, characterized in that, The construction of the water-stop curtain adopts the ultra-high pressure jet grouting method. Before excavation, the water-stop curtain is constructed in an oblique or vertical manner around the foundation pit. Before the construction of the water-stop curtain, test piles are carried out to ensure that the pile diameter, strength and verticality meet the design requirements under low clearance and limited angle.

9. The method for excavating a low-clearance open-cut tunnel under a steel box girder railway as described in claim 1, characterized in that, The entire process is automated, and real-time monitoring is performed on the deformation of existing railway tracks with crossbeams and longitudinal beams, the settlement of the abutment, the displacement of the retaining structure and the axial force of the supports. The monitoring data is directly linked to the steel support servo system.

10. The method for excavating a low-clearance open-cut tunnel under a steel box girder railway as described in claim 1, characterized in that, During the construction of the foundation pit retaining structure, a modified low-headroom drilling rig was used to construct the retaining piles.

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